1. Field of the Invention
This invention relates to the general field of biosynthesis of serine and products related to serine, particularly tryptophan, and to methods and materials used in that biosynthesis.
2. The Prior Art
Serine is a primary intermediate in the biosynthesis of a wide variety of cellular metabolites including such economically important compounds as choline, glycine, cysteine and tryptophan. In addition, serine acts as a single carbon donor and is responsible for 60% to 75% of the total need of the cell for C.sub.1 units through the production of 5,10-methylenetetrahydrofolate from tetrahydrofolate. These C.sub.1 units are used in a wide variety of biosynthetic pathways including the synthesis of methionine, inosine monophosphate, other purines and some pyrimidines (e.g., thymidine and hydroxymethyl cytidine).
The serine biosynthetic pathway shown in FIG. 1 is generally available to a wide variety of tissues and microorganisms. The first committed step in that pathway is the conversion of 3-phospho-D-glyceric acid (PGA) to 3-phosphohydroxypyruvic acid (PHA) by means of the enzyme 3-phosphoglycerate dehydrogenase (PGD). The gene encoding PGD has been cloned and sequenced, and the amino acid sequence of the PGD subunit has been deduced. Tobey and Grant, J. Biol. Chem., 261:12179-12183 (1980).
In procaryotes (particularly bacteria) and microorganisms such as yeast, but not in higher eukaryotes, activity of wild-type PG is inhibited by cellular serine levels. This inhibition has been studied kinetically and reportedly proceeds in an allosteric manner. Tobey and Grant, J. Biol. Chem., 261:12179-12183 (1986); Dubrow and Pizer, J. Biol. Chem., 252:1527-1551 (1977); McKitrick and Pizer, J. Bacteriol., 141:235-245 (1980).
Tosa and Pizer, J. Bacteriol., 106:972-982 (1971), studied the effect of a normally toxic serine analog, L-serine hydroxamate, on an E. coli strain. Selection on a growth medium containing that analog yielded serine-resistant mutants. Some mutants were shown to have a modification in an enzyme unrelated to PGD, seryl-tRNA synthetase. Crude extract of one mutant showed PGD activity with reduced serine sensitivity (See, J. Bacteriol., 106:972-982 [1971]; FIG. 5; Table 6; and see p. 973 bottom left col., p. 977 bottom left col.).